Combined cycle power plants are installations that combine the principles of a gas turbine power plant and of a steam turbine power plant. In the waste heat steam generator, the hot flue gas leaving the gas turbine is used to generate steam for the steam turbine. In that context, the transfer of heat takes place by means of a number of heating surfaces which are arranged in the form of tubes or tube bundles in the waste heat steam generator. These are in turn connected in the water-steam circuit, comprising at least one pressure stage, of the steam turbine. In that context, each pressure stage usually has, as heating surfaces, a preheater or economizer, an evaporator and a superheater.
The configuration of the waste heat steam generator is nowadays governed strictly by economic aspects. In particular, the choice of the process parameters pressure and temperature for the steam generated by the waste heat steam generator and the number of heating surfaces in the waste heat steam generator are nowadays crucial and depend both on the gas turbine outlet temperature and also the boundary conditions for operation of the combined cycle power plant. In that context, one measure for the quality of steam production at each point in the waste heat steam generator is the temperature difference between the flue gas and the steam at that point.
As a consequence of the constantly increasing demands on the economic viability of combined cycle power plants, increasing efforts have already been made hitherto, analogously to fired steam generators, to also introduce waste heat steam generators with supercritical process parameters. Since one finds oneself in this case in a pressure range above the two-phase range, in particular the natural circulation systems with their associated drums can no longer be used for the separation of mixtures, since there is no mixture of the water and steam phases with different densities. Here, the once-through principle can then be used, that is to say a high-pressure pump in the water-steam circuit conveys, in a controlled manner, precisely the right amount of water—also termed feed water—into the waste heat steam generator, such that at the outlet of the latter, in coordination with the predefined gas-side supply of heat, the corresponding quantity of steam—also termed fresh steam—emerges with the required supercritical steam parameters. Such a waste heat steam generator with at least one pressure stage operating according to the once-through principle is known for example from WO 99/01697 A1.
Since a waste heat steam generator operating according to the once-through principle requires no large-volume drums, which require thick walls in order to be strong enough to cope with the system pressure, such a steam generator is characterized by a short start-up time. A critical variable in the context of the configuration of such a waste heat steam generator is, however, still the stable through-flow of the evaporator heating surfaces over the entire load range of the combined cycle power plant.
For the purpose of increasing thermal efficiency, currently known combined cycle power plants usually have fuel preheating. This involves controlled withdrawal, from the water-steam circuit, of part of the feed water heated in the preheater, at a temperature of approximately 220° C.-240° C., at the outlet of the preheater of the intermediate-pressure stage of the waste heat steam generator, which feed water is fed to a heat exchanger circuit for fuel preheating. In that context, a suitable pressure control strategy of the intermediate-pressure stage further ensures, over the entire load range, a sufficient temperature, at this withdrawal point, of the medium circulating in the water-steam circuit.
More recent research into waste heat steam generators designed according to the once-through principle has now shown that stable flow through the evaporator of the intermediate-pressure stage can be achieved, even at the low pressures prevailing there, if the tubing of the preheater and of the evaporator is effected in one pass, that is to say without additional pressure equalization, and in the preheating region of this combined heating surface a sufficiently high pressure drop is generated. This can be ensured by designing the tubes of this heating surface with small internal diameters in the inlet region, in which exclusively subcooled water flows over the entire load range, so as to achieve the throttle pressure drop required for stable flow through the intermediate-pressure evaporator. To that end, however, it is necessary to dispense with the outlet collector at the outlet of the preheater and with the inlet distributor at the subsequent evaporator inlet. However, this does away with the branching-off line, usually provided at this point, for diverting heated water for preheating fuel. In current combined cycle power plants, omitting this fuel preheating is not desirable from the point of view of the operation of the plant as a whole. A consequence of relocating the branching-off line, for the purpose of partially diverting the preheated feed water into a heat exchanger circuit of the fuel preheating, from the outlet of the preheater of the intermediate-pressure stage to the outlet of the preheater of the high-pressure stage would be that the components of the heat exchanger circuit would have to be configured and secured for markedly higher pressures, which would in turn lead to a substantial cost increase. Switching to the low-pressure stage is not possible, since in the low-pressure stage the quantities of heat and temperatures required for the fuel preheating cannot be made available.